The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode power converters including, for instance, an asymmetrical half-bridge power converter.
A conventional asymmetrical half-bridge power converter includes two power switches coupled to a control circuit, at least one input/output isolation transformer, a voltage balancing capacitor, a rectifier and a filter. The asymmetrical half-bridge power converter generally operates as follows. The first and second power switches conduct current in a complimentary manner, with generally unequal duty cycles, to convert an input DC voltage into an AC voltage to be applied across the isolation transformer. Any DC component of the voltage applied to a primary winding of the isolation transformer is blocked by the voltage balancing capacitor coupled in series with the primary winding of the isolation transformer. The rectifier then rectifies a secondary voltage from the isolation transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load. The control circuit monitors the output voltage of the asymmetrical half-bridge power converter and adjusts the duty cycle of the power switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load.
The asymmetrical half-bridge power converter is a well known power circuit topology. The input current to a conventional asymmetrical half-bridge power converter is a discontinuous waveform that can create noise problems. The noise problems often require substantial filtering to meet specifications for conducted and radiated noise from the power converter.
Part of this discontinuity in the input current waveform occurs due to currents in various magnetizing and output inductances being switched by the first and second power switches, thereby generating AC components in the input current. In addition, the output current reflected to the primary winding and alternately switched by the first and second power switches generates a significant component of the input ripple current. To address the potentially deleterious input current ripple, a magnetizing inductance of the transformer may be made larger by minimally gapping the transformer core, consistent with the requirement to prevent core saturation by a DC component of the magnetizing flux. Additionally, an output inductor may be made as large as practical, consistent with design trade-offs. Although helpful, attention to these considerations does not sufficiently reduce the input ripple current.
The input ripple current also varies in magnitude as a function of the duty cycles of the power switches. At a duty cycle of 50 percent, the input ripple current is typically at a minimum, and is primarily composed of the switched current in the transformer magnetizing inductance. However, the asymmetrical half-bridge power converter (and many other types of power converters) is not typically operated at a 50 percent duty cycle, thereby resulting in input ripple currents that are higher than desired.
Accordingly, what is needed in the art is an improved way to reduce input ripple current in an asymmetrical half-bridge power converter.